Download the appropriate version of BitTorrent Sync and open a terminal on your Download directory. The archives have various names, so I’ll just use a generic one. First, extract the executable.

tar xf btsync.tar.gz

Then, move it to the user binaries folder.

sudo mv btsync /usr/bin

Now, open up your scheduling configuration.

crontab -e

To the bottom of the file, add this:

@reboot btsync --config ~/.config/btsync/btsync.json

Exit the editor. Now, create the folder and the file. Choose a port and change it to something you like (the “listen” attribute under “webui”). You may also add “username” and “password” to this, although you don’t have to. These will be configurable later through the UI.
Important: the file paths in the config file have to be absolute, don’t use the ~ variable here!

mkdir -p ~/.config/btsync
sensible-editor ~/.config/btsync/btsync.json

{
    "storage_path": "/home/username/.config/btsync",
    "pid_file": "/home/username/.config/btsync/btsync.pid",
    "webui":
    {
        "listen": "0.0.0.0:51920"
    }
}

After you’re done, simply reboot to work the magic. Or execute btsync with its --config parameter.